This thesis describes a second-generation walkthrough framework that provides extensive facilities for integrating many types of third-party simulation codes into a large-scale virtual environment model, and puts it in perspective with first-generation systems built during the last two decades. The framework provides an advanced model database that supports multiple simultaneous users with full consistency semantics, system independent storage and retrieval, and efficient prefetching and object reconstruction techniques to support second and third-generation walkthrough systems. Furthermore, our framework integrates support for scalable, distributed, interactive models with plug-in physical simulation to provide a large and rich environment suitable for architectural evaluation and training applications. A number of third-party simulations have been integrated into the framework, including dynamic physical interactions, fire simulation, multiple distributed users, radiosity, and online tapestry generation. All of these simulators interact with each other and with the user via a data distribution network that provides efficient, optimized use of bandwidth to transport simulation results to clients as they need them for visualization. These diverse simulators provide proof of concept for the generality of the framework, and show how quickly third-party simulations can be integrated into our system. The result is a highly interactive distributed architectural model with applications in research, training, and real-time data visualization. Finally, an outlook is given to a possible third generation of virtual environment architectures that are capable of integrating different heterogeneous walkthrough models.